1. Field of the Invention
Some preferred embodiments of the present invention relate to a capacitance discrimination circuit configured to discriminate capacitance added to a terminal, a touch switch equipped with the capacitance discrimination circuit, and a capacitance discrimination method.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As an input device for various electronics devices, such as, e.g., a mobile phone, a mobile audio device, a handheld gaming gadget, a television, or a personal computer, conventionally known is a device called a touch switch or a touch sensor equipped with electrode switches (hereinafter simply referred to as “touch switch” in this document) configured to discriminate whether or not the switch is being touched by a finger of a user by detecting capacitance changes thereof. For example, this kind of input device is disclosed by Japanese Unexamined Laid-open Patent Application Publication No. 2000-65514 (JP-A-2000-65514).
In such a conventional touch switch configured to detect capacitance changes, it is discriminated whether or not the touch switch, more specifically an electrode switch provided on a touch panel, is being touched by a finger of a user by measuring a voltage of a capacitor attached to a terminal (electrode switch).
FIG. 5 is an explanatory view of an operation of a touch switch. This touch switch has two electrode switches 1a and 1b on a touch panel P, and is configured to discriminate whether or not the electrode switch is being touched by a finger F of a user. In this switch, since a finger F of a user has a capacitance C2′, when a finger F touches the electrode switch 1b, the capacitance C2′ of the finger F is added to the capacitance C2 of the electrode switch 1b. In this case, as compared with the electrode switch is to which no finger is being touched, the electrode switch 1b becomes larger in capacitance.
FIG. 6 shows voltage increase changes of two capacitors C1 and C2 different in capacitance. As shown in this figure, these capacitors C1 and C2 are different in voltage increase rate, causing a difference in time required for each capacitor to reach a reference voltage Vref, which enables discrimination of a difference of capacitance in these capacitors C1 and C2. At this time, provided that the capacitor C1 and the capacitor C2 are designed to have the same capacitance, since the apparent capacitance of the capacitor C2 is larger than that of the capacitor C1, it can be discriminated that the electrode switch having the larger apparent capacitance C2 is being touched by a finger of a user. This principle can be utilized in a touch switch.
In the aforementioned conventional touch switch configured to detect capacitance changes, however, the time required for the capacitor to reach the reference voltage Vref was simply measured, and therefore there was a problem that the measured time was readily affected by external noises.
For example, in some cases, external noises may cause a slower voltage increase rate of the electrode switch even if the switch is not being touched by a finger of a user. In such a case, the conventional touch switch may sometimes recognize that the slower voltage increase rate was caused by a touch of a user's finger to the electrode switch, and may erroneously discriminate that the touch switch is being touched by a finger of a user.
FIG. 7 is a graph showing voltage changes of a capacitor in which the solid line shows a voltage change in cases where the capacitor is not affected by noise and the dashed-line shows a voltage change in cases where the capacitor is affected by noise. In cases where the capacitor is not affected by noise, as shown in the solid line, as electric charges are added gradually, the voltage increases smoothly. On the other hand, in cases where the capacitor is affected by noise, as shown in the dashed line, even if electric charges are added gradually, the voltage does not increase smoothly, resulting in unsteady voltage increase, which causes erroneous discrimination.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.